We show that SnO2 nanoclusters in silica interact with ultrashort infrared laser pulses focused inside the material generating a hydrostatic compression and photoelastic response of the surrounding glass. This effect, together with the laser-induced nanocluster amorphization, gives rise to positive or negative refractive-index changes, up to 10–2, depending on the beam-power density. This result points out a wide tuning of the refractive index patterns obtainable in silica-based optical technology

A. Lauria

A. Paleari

Bricchi E.

E. Bricchi

E. Franchina

N. Chiodini

P. G. Kazansky

English

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SnO2 nanoparticles in silica: Nanosized tools for femtosecond-laser machining of refractive index patterns

Paleari, A.

Franchina, E.

Chiodini, N.

Lauria, A.

Bricchi, E.

Kazansky, P.G.

Southampton (e-Prints Soton)

APPLIED PHYSICS LETTERS 88, 131912 2006SnO2 nanoparticles in silica: Nanosized tools for femtosecond-lasermachining of refractive index patternsA. Paleari,a E. Franchina, N. Chiodini, and A. LauriaCNISM and Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 53,I-20125 Milano, ItalyE. Bricchi and P. G. KazanskyOptoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United KingdomReceived 22 July 2005; accepted 11 March 2006; published online 31 March 2006We show that SnO2 nanoclusters in silica interact with ultrashort infrared laser pulses focused insidethe material generating a hydrostatic compression and photoelastic response of the surroundingglass. This effect, together with the laser-induced nanocluster amorphization, gives rise to positiveor negative refractive-index changes, up to 10−2, depending on the beam-power density. This resultpoints out a wide tuning of the refractive index patterns obtainable in silica-based opticaltechnology. © 2006 American Institute of Physics. DOI: 10.1063/1.2192579In the last decade, the ability of lasers to induce a modi-fication of the optical properties in silica-based materials be-came matter of extensive investigation.1–7 Indeed, a photo-induced modification of the refractive index can be utilizedto create waveguides, gratings, and other optical patterns forintegrated photonic devices in a one-step technique. In thisframework, the photorefractive response of a nanostructuredcomposite may be drastically innovative. In fact, differentresponses of coexisting phases to the laser radiation are ex-pected to give rise to concomitant and possibly interactingprocesses to achieve a wide tuning of refractive index. Wide-band-gap nanophases in silica are promising systems in thisregard, since their optical properties would allow the lasermachining inside the bulk. In such a system, i.e., a silicacomposite containing SnO2 nanocrystals,8 we recently founda negative photosensitivity to unfocused ultraviolet and vis-ible lasers.9,10 In this work we demonstrate the feasibility ofwriting a buried pattern of refractive index n with eithernegative or positive n by varying the power density of afocused beam of an infrared femtosecond laser.We have analyzed femtosecond-laser writing processesin nanostructured optical grade SiO2:SnO2 glass ceramics84:16 molar ratio synthesized from a sol-gel route,8 to-gether with a reference silica sample. An amplified mode-locked Ti:sapphire laser operating at 800 nm 200 fs pulseduration was used to irradiate the samples. Squares of 100100 m2 were written at about 250 m from the surface,at different pulse energy values, by writing adjacent linesspaced 1 m apart at a speed of 80 m/s with the beampolarization orthogonal to the lines. Two focusing configura-tions were used with high and low numerical aperturesNAs 0.55 and 0.21, respectively. Images of the writtenpatterns, in unpolarized light and through two crossed polar-izers, are reported in Fig. 1.The laser-induced modification n with respect to theunprocessed bulk was measured from the change of opticalpath experienced by a polarized probe beam at 633 nm ontraversing the sample. Possible laser-induced birefringence7was investigated analyzing the phase xy and xx for theaElectronic mail: alberto.paleari@mater.unimib.it0003-6951/2006/8813/131912/3/$23.00 88, 13191Downloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to orthogonal and parallel directions with respect to the writingpolarization. Structural modifications along the z axis of laserpropagation were analyzed by confocal micro-Raman spec-troscopy. Changes of nanometric structure were analyzed bytransmission electron microscopy TEM.Figure 2 summarizes the interferometry results. Thenegative  in glass ceramic after low-NA irradiation isFIG. 1. Color online a Optical microscopy image of a refractive indexpattern photowritten in SnO2:SiO2 glass ceramic. The white-drawn pictureis a schema of the high-NA dotted line and low-NA straight line configu-rations used for focusing the laser beam at the focal layer FL. b Directwritten patterns processed at different pulse energies: 1.5, 1.25, and 1 Jfrom left to right, with unpolarized light top and with the sample betweencrossed polarizers bottom.© 2006 American Institute of Physics2-1AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp131912-2 Paleari et al. Appl. Phys. Lett. 88, 131912 2006consistent with the negative photosensitivity previouslyobserved with unfocused lasers.9,10 Remarkably,  changessign accordingly to the focusing conditions. No such achange occurs in glass, where negative  and large bire-fringence are observed, regardless the NA value, in agree-ment with previous results.4,7Figure 3a shows Raman spectra at different depthsfrom the high-NA irradiated surface of the glass ceramic.FIG. 2. Phase shift, for polarization parallel filled marks and orthogonalopen marks to the polarization of the writing laser, due to laser-inducedrefractive index changes in glass triangles and SnO2 containing glasscircles, at a high-NA and b low-NA focusing of the writing beam vs thelaser pulse energy. c Phase shift in glassceramics vs the estimated relativevalue of maximum energy density per pulse along the propagation axisinside the material.FIG. 3. a Confocal micro-Raman spectra at 50 lower curve and 250 mupper curve from the sample surface after femtosecond writing at 1.25 Jof energy pulse. Inset: difference spectrum. b Amplitude decrease of theA1g mode of crystalline SnO2 and c energy of the 1 silica mode at in-creasing depth from the front surface of the material.Downloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to The A1g mode of SnO2 Ref. 11 decreases, approaching thefocal plane Fig. 3b, while a broadband grows at 560 cm−1inset of Fig. 3a ascribable to amorphous SnO2,9,11 and thesilica 1 band12 shifts at high energy Fig. 3c. These dataindicate a strongly modified buried region of 50 m,where the fraction of crystalline nanophase is reduced in fa-vor of an amorphous phase. No detectable change of the A1gmode is observed at low NA. The reference glass does notdisplay any change.TEM images of the unprocessed material Fig. 4ashow SnO2 single domain nanocrystals of about 10–15 nmhomogeneously dispersed in the glass. However, by sam-pling the laser-irradiated material, alongside the regions withstandard size clusters, micrometer regions containing onlynanoclusters smaller than 6 nm are observed Fig. 4b.Several samplings in the two kinds of regions show distinctsize distributions histogram in Fig. 4.These results suggest the following comments. The posi-tive phase-shift across the sample written in tighter focusingconditions cannot be interpreted as the superposition of theindependent responses of the coexistent phases, both beingnegative. A model of the interaction between nanophase andsilica can be rationalized considering that the decrease ofnanocrystal size implies a transformation in an amorphousphase. This kind of transformation should be accompaniedby a volume expansion Vnp. Indeed, the estimated9 refrac-tive index of laser-induced amorphous SnO2 na=1.89 issmaller than nc=1.99 of the crystalline form, consistent witha lower density. The increase of volume of the nanoparticleinduces a compression and eventually a compaction in thesurrounding silica host, with a positive contribution to n.FIG. 4. a TEM image of SnO2 nanoparticles in 16 mol % SnO2:SiO2 glassceramic. Inset: High resolution TEM image of a SnO2 nanoparticle. bReduced-size and c unperturbed-size nanoparticles in laser-processedand -unprocessed regions, respectively, of the material. Histogram: statisticsof cluster sizes from sampling of laser-modified white barsand -unmodified black bars regions.The shift 1 indeed confirms the presence of compressiveAIP license or copyright; see http://apl.aip.org/apl/copyright.jsp131912-3 Paleari et al. Appl. Phys. Lett. 88, 131912 2006strains into the glass. Nevertheless, we have also to considerthat the host resists against the compression and applies inturn a compression to the nanoparticles.In an attempt to analyze this process, we can quantifythe change n of refractive index in the modified regionfrom the phase shift . From the relation =2nd / +2 rad Fig. 2a, taking a thickness d=50 m Fig. 3b, we obtain approximately n=410−3.The decrease of the molar fraction y of crystalline SnO2 inthe focal layer with respect to the starting value y0=0.16 canin turn be obtained from Raman and TEM analysis. The low-ering of the A1g Raman mode suggests a 50% decrease Fig.3b. Really, an even higher decrease is expected from anonperfect rejection of the Raman signal from unperturbedregions. The TEM analysis in fact suggests a 98% reductionof the crystalline phase. In this case, the refractive indexchange n is approximately given by the combination of thechanges nnp and ns in the nanophase and in the surround-ing silica, respectively, weighted by the relative phase abun-dance. The description of the changes nnp and ns can betreated through the Lorentz-Lorenz LL equation13 n= n2−1n2+2 /6n /− V /V for both the silicamatrix and the nanophase, where V /V is the relative vol-ume change, comprising both elastic compression andirreversible compaction contributions, and  / the pos-sible change of ionic polarizability. If we assume that theoverall volume of the sample does not change under irradia-tion, the change of volume of the nanoparticle is compen-sated by an opposite change of volume of the surroundingsilica Vs=−Vnp. In the LL relation, np/np is mainlydetermined by the structural transformation involved in thenanoparticle amorphization. This process should involve achange of Sn–O coordination, since no sixfold-coordinatedamorphous form of oxides is known. np/np can be esti-mated looking at analogous group-IV oxides, SiO2 andGeO2, both occurring as fourfold-coordinated glass andsixfold-coordinated crystals. In both cases,  / is 11%.14If we assume this value and we take9 nnp=−0.1 as trialvalue in the LL relation, we finally obtain Vnp/Vnp=18%,corresponding to a silica volume change Vs /Vs=−3%.Smaller values of nnp and Vnp/Vnp are probably morelikely, accounting for the host resistance against the nanopar-ticle expansion. From the experimental n and taking 15%	 Vnp/Vnp	18%, we obtain s /s / Vs /Vs between0.33 and 0.22, respectively. This ratio is 0.33 for a purephotoelastic response and 0.19 for UV-induced silicacompaction.15 This suggests that the process differs from theUV-induced compaction of silica, specifically for a largerDownloaded 11 Nov 2009 to 152.78.208.72. Redistribution subject to contribution of elastic compression ascribable to the reactionagainst the nanophase amorphization.As regards birefringence, in glass ceramics it is smallerthan in glass, negligible at low NA. The mechanism occur-ring in glass, responsible for a form birefringence,7 is indeedinhibited in glass ceramics because the radiation is preva-lently absorbed by nanoparticles, making multiphoton eventsof glass ionization much more rare.Finally we consider the origin of the different responsesin high- and low-NA configurations. Since the radiationwavelength lies at 800 nm, the observed effects imply two-photon absorption with a 2 dependence on the pulse energydensity . An approximate calculation of  as a function ofthe depth z from the surface and the pulse energy E can becarried out accounting for the increase of  due to the fo-cused geometry and the decrease due to nonlinear absorp-tion. Within this approximation, the maximum  achievablealong the z axis is proportional to NA2E / 1−NA21/3.High-NA and low-NA data, plotted together as a function ofthis parameter Fig. 2c, indeed disclose a single continu-ous response behavior.In summary, SnO2 nanoparticles trigger a chain of physi-cal mechanisms activated by the interaction with IRfemtosecond-laser pulses, providing a flexible tool for thecontrolled machining of refractive index patterns in silica.This work was supported by the Italian MIUR.1C. B. Shaffer, J. F. Garcia, and E. Mazur, Appl. Phys. A: Mater. Sci.Process. 76, 351 2003.2P. Niay, M. Douay, P. Bernage, W. X. Xie, B. Leconte, D. Ramecourt, E.Delevaque, J. F. Bayon, H. Poignant, and B. Poumellec, Opt. Mater. Am-sterdam, Neth. 11, 115 1999.3J. Canning, K. Sommer, M. Englund, and S. Huntington, Adv. Mater.Weinheim, Ger. 13, 970 2001.4Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, Phys. Rev. Lett. 91,247405 2003.5K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao, Appl. Phys. Lett.71, 3329 1997.6J. D. Mills, P. G. Kazansky, E. Bricchi, and J. J. Baumberg, Appl. Phys.Lett. 81, 196 2002.7E. Bricchi, B. G. Klappauf, and P. G. Kazansky, Opt. Lett. 11, 115 2004.8N. Chiodini, A. Paleari, D. Di Martino, and G. Spinolo, Appl. Phys. Lett.81, 1702 2002.9N. Chiodini, A. Paleari, and G. Spinolo, Phys. Rev. Lett. 90, 0555072003.10N. Chiodini, A. Paleari, G. Spinolo, and P. Crespi, J. Non-Cryst. Solids322, 266 2003.11A. Dieguez, A. Romano-Rodriguez, A. Vila, and J. R. Morante, J. Appl.Phys. 90, 1550 2001.12F. L. Galeener, Phys. Rev. B 19, 4292 1979.13C. Fiori and R. A. B. Devine, Phys. Rev. B 33, 2972 1986.14J. A. Duffy, J. Phys. Chem. B 108, 14137 2004.15R. E. Schenker and W. G. Oldham, J. Appl. Phys. 82, 1065 1997.AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

SnO<sub>2</sub> nanoparticles in silica: nanosized tools for femtosecond-laser machining of refractive index patterns

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SnO<sub>2</sub> nanoparticles in silica: nanosized tools for femtosecond-laser machining of refractive index patterns

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